'Best Of' archive
GM3SEK's Amateur Radio
Further information about recent articles
For older information, check the separate Cumulative
Index and 'Best Of' pages.
Cost-effective Ferrite Chokes and Baluns (May 2010)
Original design concept for these chokes, published
in the 2010 ARRL Handbook, was by George Cutsogeorge, W2VJN.
Other Ferrite Cores
If you are ordering
other types of cores, Fair-Rite
31 material is probably the best grade for general purpose HF and low-band
chokes. See Jim Brown, K9YC's references below.
THE PROBLEM SOLVER - Fair-Rite
31 clamp-on core, Part No 0431177081
This is a serious ferrite core for RFI
problem-solving - not only because it can make a highly effective choke at HF, but also because it clamps onto an existing run of cable without needing to remove connectors. (Remember the 13th Commandment: "Thou shalt not cut off thy neighbour's mains plug".)
But also remember that an HF ferrite choke will typically need 6-8 turns passing through the centre hole. Regardless of the type of core, one pass through the centre will never be effective at HF!
About 8 turns of 3-core mains cable on this specific core is now my best
recommendation for a whole-shack mains filter (the old Belling-Lee three-wire filter is no longer available).
Mouser is currently the best deal for this large clamp-on core, because FedEx shipping to UK for
orders over £50 is FREE! (uk.mouser.com is like a normal UK-based order
page - you'll be charged VAT in the usual way, and then the parcel arrives
in about 3 days with nothing more to pay.)
This link takes you directly to the correct core.
The same Fair-Rite 31 material is also available in a "FT240" 2.4-inch toroid format
Fair-Rite Part No 2631803802. 10-12 turns of coax on a FT240-31 makes a nice low-bands choke
Mouser's free FedEx offer is currently the best deal.
This link takes you directly to the correct part number (ignore
- Common Mode Chokes –
neat graphical presentation of impedance measurements on a wide
range of chokes, clearly showing the impedances and bandwidths
reactive (air core) chokes are undesirable – they can sometimes
make common mode currents worse.
- Jim Brown,
- Chuck Counselman,
- Common Mode Chokes – how one man transformed his ‘noisy’ location
to a quiet one, using heroic quantities of ferrite.
- Owen Duffy,
Care and Feeding of VHF/UHF Long Yagis
Websites about Yagi design
For more links, references and construction ideas,
see the VHF/UHF Long Yagi Workshop
pages here on this site.
'1/12th wave' Impedance transformer
using alternating coax impedances (Figure 3b)
Loctite Threadlocking User's Guide
Google for lots of UK suppliers, eg
the semi-solid Loctite 248 in the 'lipstick' package. (I haven't tried
this, but prefer the liquids because they run into the threads so easily.)
It was a real squeeze to fit the basic story into the two
pages, so there's quite a lot of web-only content this month.
notes and references
There is a better
picture of all the capacitors on the Radcom Contents page.
to the October 2009 column (on the RSGB Members-Only site)... coming
when the RSGB webmaster gets round to it.
 Although Z1 and
ZC1 are both complex (vector) impedances, the important criterion
is that |Z1| needs to be much greater than |ZC1|. If this is
achieved, a detailed vector calculation is unnecessary.
The models in Figure
3 (page 80) work quite well with simple fixed values of C and Ls, at
least for values of C up to about 100nF. However, Rs should be modelled
as a function of frequency, particularly near the series and parallel resonances
which are quite sensitive to the values of Rs.
Also, do you see a resemblance
between the green traces in Figure 6 and Figure 4? It suggests
that the 100nF 1kV capacitor (top right in the photo on the Contents page)
may actually contain two different capacitors in parallel.
Page 81, bottom
of column 1: should refer to Figure 4.
plots and discussion
These plots show
what can happen if you attempt to damp out an unwanted parallel resonance
that doesn't actually exist.
Both plots below
are for the 10uF electrolytic and 10nF ceramic capacitor in parallel.
Figure 6 on page 81 shows no sign of parallel resonance. (It is
barely visible as a wobble around 8MHz, but is thoroughly damped by the
internal resistances of these particular components.)
Both of these
so-called 'cures' have been recommended in various books, and they only
make things worse!
resistor in series with the electrolytic capacitor:
Comment: this has no benefit at all - it only increases
the value of Rs at low frequencies.
on one lead of the electrolytic
a very bad idea - it creates exactly the kind of resonance it was
supposed to prevent!
If you don't have the test equipment to see what could
be going wrong,
it's best to keep it simple.
Errors in VSWR Meters
Current versus Voltage baluns
'Low Noise' Yagis
RSGBtech mailing list
attachments are public. To contribute and use other facilities, you need
to join the group.
Metal Film Resistors
this to demonstrate 'zero lead length' construction for improved
VHF performance. The VSWR is excellent up to 50MHz, limited mostly
by the quality of the coax connector termination. The effects of
shunt capacitance in the TO220 packages (about 2pF) become more noticeable
above 50MHz but this load is good for most purposes up to 144MHz.
and wrist straps
Search eBay.co.uk for esd mat, then sort through the list for
This link should take you there:
Look for mats of about 500x600mm that include a wrist strap.
stories from Doug Smith's
website - a mine of useful information about RF measurements, EMC
inside ICs - an interesting
general review in the opening chapters.
Complex Impedance, VSWR and Reflection Coefficient
many sites and programs that convert between VSWR, reflection coefficient
and return loss, but most of them don't allow complex impedance. Here
are a few that do:
Can you explain the small difference in VSWR between
the two programs?
(Hint: both programs have calculated correctly.)
A Filtered Mains Supply for Your Shack